Since silicon carbides (SiC) have a larger band gap than silicon and a dielectric breakdown field strength about ten times as large as silicon, silicon carbides have been used in various semiconductor elements including power semiconductors. About 200 types of crystals have been known for SiC around those of 3C-SiC, 4h-SiC, and 6h-SiC. Among them, 4h-SiC has been used generally since a band gap is as large as about 3.2 eV and a substrate can be prepared more easily compared with other structures. As the semiconductor devices, 4h-SiC is mainly used for power diodes and power MOSFETs (Metal Oxide Field Effect Transistors). Among them, SiC power MOSFET has higher switching speed since this is a unipolar device, when compared with Si IGBT, and the substrate can be made thinner since it has higher dielectric breakdown field strength and the resistance during operation referred to as on resistance can be lowered when compared with the Si power MOSFET.
FIG. 2 shows a cross sectional view of a typical power MOSFET. In 4h-SiC, (0001) surface where the c-axis is perpendicular to a substrate allowing easy manufacture of the substrate is used for the surface orientation of substrate. The drain region 2a at the back of the substrate is in contact with a drain electrode 1a at a high concentration of about 1018 (cm−3) in order to lower the contact resistance. Further, the drain region 2a and an n-drift region 3a at a low concentration are prepared from the drain region separately by epitaxial growth. A base region 4a comprises a p-type impurity layer, in which an n-type inversion layer is formed just below a gate oxide film 7a when the gate electrode 6a is turned on and in electric conduction with a source region 5a. 
The power MOSFET also includes a trench type as shown in FIG. 3. In the structure, an additional step is necessary for forming a trench compared with the plane type power MOSFET. However, since a channel is formed in a direction perpendicular to the substrate, refinement is easy, and the on resistance can be lowered and the chip area can be decreased by improving the channel density. Further, it does not cause JFET (Junction Field Effect Transistor) resistance due to a depletion layer in the junction between the drift layer 3a and the p-type impurity layer 4a on both sides thereof just below the gate oxide film 7a of FIG. 2. Further, since the electron mobility in 4h-SiC is greatest along the direction parallel to the c-axis, the channel mobility increases and the channel resistance is decreased in the trench type MOSFET having the substrate surface of (0001) surface.
By the way, while the ratio of the channel resistance to the entire on resistance in the SiC power MOSFET decreases as the designed withstanding voltage is higher, this is generally larger when compared with the power MOSFET or Si IGBT. This is because SiC has high a withstanding voltage, the thickness of the drift layer can be reduced to about 1/10 compared with that of the Si element and, since the resistance of the drift layer is low and, on the other hand, the rate of decreasing the channel mobility than the bulk mobility is greater than that in the Si element.
Accordingly, for lowering the on resistance of the SiC power MOSFET, it is necessary to lower the channel resistance.
For the subject, Japanese Patent Unexamined Application Publication No. 2005-244180 describes a method of decreasing the effective mass of carriers by applying a tensile strain in a uniaxial direction to thereby change the band mass and to improve the channel mobility.